The present invention relates generally to high speed data communications and, more specifically, to a system and method for uplink power control in a third generation (3G) code division multiple access (CDMA) communications system used in conjunction with message retransmission protocols.
In second generation CDMA systems, message size for reverse link data bursts was limited, typically to 8 frames or less, to prevent cell sensitivity degradation. In IS-2000A, a new access channel is defined, the R-CCCH, which permits the transmission of very long data bursts or messages (in excess of 200 frames) at high rate (up to 38.4 kbps) over a reserved, power-controlled channel of the CDMA system. More specifically, the R-CCCH is a reserved (i.e. contention-less) whose framing format includes 5, 10, or 20 msec frames that can be transmitted at 9.6, 19.2, or 38.4 kbps. The R-CCCH is power-controlled by a sub-channel of the forward common power control channel (F-CPCCH). The procedure to reserve the R-CCCH takes place on a different set of common channels, namely the R-EACH and the F-CACH. The R-CCCH was introduced in IS-2000A because it provides significant advantages with respect to the R-EACH, both in terms of capacity and throughput, especially for longer messages ( greater than 200 msec).
The data bursts or messages transmitted over the R-CCCH consist of multiple frames. Transmission of the data bursts or messages is made reliable by using transmission repetition protocols such as stop-and-wait or selective repeat. Due to the protocol overhead, however, the retransmission protocol is implemented at message level rather than at frame level. That is, upon successful message decoding, the base station acknowledges receipt of the entire message, but not of any of its constituent frames. Thus, if any frame of the message was unsuccessfully decoded, the entire message must be retransmitted.
After the receipt of the last transmitted frame of the data burst, the base station attempts to re-assemble the message. If the message contains no errors as indicated by the error detecting decoder, the base station transmits an acknowledgment message over a forward control channel to the mobile station. Conversely, if message re-assembly fails (a situation indicative of not having received all the frames correctly), then the base station may take no action. The mobile station waits for the base station acknowledgement and, if the acknowledgment is not received within a given amount of time or if a negative acknowledgment is received, the mobile station retransmits the entire data burst message.
The R-CCCH is power controlled by the base station during message transmission. The base station transmits a sequence of power control bits to the mobile station over the forward power control channel. The polarity of the power control channel bits indicates either an xe2x80x98upxe2x80x99 or xe2x80x98downxe2x80x99 command. The base station periodically estimates the received energy of the reverse control channel and transmits an xe2x80x98upxe2x80x99 command if the energy falls below a signal to noise threshold, and transmits a xe2x80x98downxe2x80x99 command otherwise. The mobile station receives the power control bits and adjusts its reverse control channel transmit power accordingly. The function outlined above is usually called inner loop power control, and it is equivalent to the one normally used for the dedicated traffic channel of IS-95B CDMA systems.
There are some key issues that, unless addressed appropriately, may lead to the degradation of both system capacity and throughput performance of the data burst protocol over the reserved channel. The first key issue is the fact that there is no provision for a selective retransmission procedure on a frame level. Rather, if one or more frames are received in error, message capsule re-assembly fails and the data burst must be retransmitted entirely. Thus, for longer messages, the data burst signal must be received at a signal to noise ratio that is considerably higher than that required for a dedicated traffic channel operating at the same data rate, where the target frame error rate of the traffic channel is the same as the reserved channel target message error rate. Assuming independence of the received frame erasures, the target message error rate (MER) has a relationship to the frame error rate (FER) that is a function of the message size, where N is the number of frames in the message:   MER  =                    1        -                              (                          1              -              FER                        )                    N                    →      FER        =                  1        -                              (                          1              -              MER                        )                                1            N                              ≈              MER        N            
The result above indicates that when the message size is equal to 100 frames, for example, the reserved required FER is approximately 100 times smaller than that required for the dedicated channel FER. A required FER ratio equal to 100 corresponds to an increased required signal to noise ratio of 3 to 9 dB, approximately, depending on channel conditions. If the reserved channel sensitivity is much higher than that of the dedicated channels, the data burst transmission will cause a temporary cell sensitivity degradation. That is, the cell maximum path loss (cell size) will shrink by the same amount, possibly causing an outage for all the users near the cell boundaries. Of course, the cell capacity is also momentarily degraded.
A second key issue is the selection of the power control inner loop set point. The inner loop set point required for reliable message detection may depend on channel conditions and on the message size, which may not be known a priori to the base station. Without an outer loop function, the inner loop set point must be set to a conservative value corresponding to the worst case (i.e., to the maximum allowable message size and worst case channel conditions). If the set point is set to a value less than the value corresponding to the worst case, long messages and/or messages transmitted over a poor channel are likely to be retransmitted several times. This may compromise the throughput and capacity of the common channel. However, when the set point is set to a value corresponding to the worst case, considerable capacity is wasted for transmission the of any message with a length less than the maximum length and/or messages transmitted over a favorable channel.
The considerations above clearly demonstrate the advantages that an outer loop function can provide with respect to the throughput and capacity of data burst messages. In particular, the outer loop function can adjust the required inner loop set point, and so achieve the desired MER with the least possible capacity consumption. Conventional outer loop functions, such as the one used for the dedicated channels, are driven by frame erasures or, more generally, by some quality metric of the received frame. That is, they are driven by some small acceptable error rate in an attempt to set the mobile station transmit power to the lowest possible level to support communications. However, this conventional outer loop function, based on an acceptable error rate, cannot be applied to the burst transmission over reserved channels because a single frame received in error will cause the message to be discarded, and require the mobile station""s retransmission of the entire message. Thus, there remains a need for improved power control methods suitable for use with message retransmission protocols to adjust the inner loop power control set point.
The present invention provides a system and method of power control useful for message retransmission protocols to prevent unnecessary degradation of cell sensitivity. A transmitting station transmits a message comprising a plurality of data frames to a receiving station over a reverse common control channel. The receiving station controls the power level of the transmitting station by sending power control commands to the transmitting station during message transmission. The receiving station measures the energy of the received signal and computes a signal to noise ratio, which the receiving station compares to a target signal to noise ratio referred to herein as the power control set point. The receiving station transmits power control bits to the transmitting station to signal the transmitting station to either increase or decrease the transmit power level based on the comparison, and the transmitting station adjusts its transmit power accordingly.
The receiving station re-assembles the message after receipt of the last frame and, it the message contains no errors, transmits an acknowledgement to the transmitting station over a forward control channel. If the message reassembly fails, the receiving station saves correctly received frames in a reassembly cache and waits for the message to be re-transmitted by the transmitting station. After transmitting the message, the transmitting station waits for an acknowledgement from the receiving station for a predetermined time period. If an acknowledgement is not received, the transmitting station repeats the entire message.
At the beginning of each message transmission, the outer loop power control in the receiving station calculates the power control set point used for inner loop power control based on the effective message length. The effective message length is the number of frames that are yet to be correctly received. If the message length is not known, a default value may be assumed for the initial transmission of the message. The power control set point is directly proportional to the effective message length so that the power control set point decreases as the effective message length decreases. The net effect of the inner loop and outer loop power control functions is a reduction in transmit power as the effective message length decreases.
The power control set point used for inner loop power control can be further adjusted on a frame by frame basis during each message transmission. The receiving station decodes each frame as it is received. The receiving station adjusts the power control set point downwardly if the frame is correctly received, and adjusts the set point upwardly if the frame is not correctly received. The step size for upward adjustments may be greater than the step size for downward adjustments.